Glasses, compositions, thereof, articles embodying said glasses, and method of making and using the same



'RfkafBAR-NES .E1-'AL v CLASSES, cQMPosmoNs THm-:QHARTICLES .waonyrmc SAID GMS'SES,l AND OF MAKING VAND USING THF 'SAME YFiled May 13, 1950 5 Sheets-Sheet 3 ver;v an

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laurea A. PR-ER GLASSES, COMPOSITICNSTHERsoF, ARTljcLEjs EMBQDYINGSA'ID l GLASSES, 'I'NDJL'ZTHOD OF MAKING AND USING THE SAME Filed May 13, 1950 5 SheetsQSheet 4 Oct. 13, 1953 A l lR -'NEs EAL f 655,452 k' eo V WAVELENGTH soo zo Oct. 13, 1953 RB. BARNES ,Er AL 2,655,452

sLgssas. composwzous mamon Ammss wonymc 'smv f GLASSES, AND-xmms o? MAKING- AND USING- THF; sms Filed may 15, 195o Y s sheets-sheet s 3 of elect-rens toward said face par-tion. The coneshaped Walls 4 may be formed of glassnietal, or other desirable material in .the conventional manner with the glassface portion' 3' being secured in sealed relation thereto.

The face portion 3'- formed of the vglass mbodying the invention has a -uorescent screen l formed either directly on the inner surface thereof ron a coating 8 fof transparent in aterial disposed on the inner surface .oi the glass. In the latter instance, the transparent coating 8 is adapted to space the screen lfrom the inner surface of the face portion 3- and the function thereof will be described in detail hereinafter. A reflection reduction coating 6 may be formed o n the outer surface of the face portion of the tube.

It has been described above that .the conical wall portions 4 may be formed of glass. In such instances', however, the glass of said wall 'portions may be those which are now in common commercial use and need not be of the glass' em.- bodying the invention.

In dealing with thevarious'idiniculties set forth above, `the glass einbodying the invention preferably has the followingcharacteristics:

1. Transnzission in the visible portion of the spectrum ranging approximatelyfrom 45% to 70% at 400 niillimierons, 58% to 76% at 500 mile limicrons, 50% to 71% at 600 millirnicrons, and 67% to 80% at 700' millimiorons-and with an average White of f-ronbi to 75%.

2. 'Iransrnission'in the ultra-violet portion of the spectrum of approximately from 2% to 31%. at 370 milliinicrons, approximately from to 13% at 360 millimicrons, approiniately from. 0% o 3.5% at 350 millimicron's, and with substantially no. transmission below 350. miilixnicrons.

3. Absorption in theX-ray portion ofthe spec- .trum of st ibstantially4 all X- rays generated .thereby These transmission and absorption values apply to glasses embodying the invention whenV produce-'d and measured in thicknesses of from to (i1/millimeters However, in certain instances .it may be permissible to v'a'ry the above specifications in accordance with the results desired and the' thickness of the article.

According to the teachings of this invention, a soda-potasliiime silica base glass batch will have added thereto controlled amounts of the oxidesoflead, manganese, vanadium, copper and silver. for obtaining the desired color andv ultra-violet absorption characteristics in the resultant glass. Inclusion of ooi-ic acidivill also aid in the incitiing, making the glass Amore fluid and improving. the chemical durability.'

. The following tab-le lists the ingredients which may be used in producing a batch which when meltedin the manner to be described hereinafter will result in a crown glass having y desired characteristics:

v Table A Amounts,

lngdm. percent Manganese oxide (MnO:)... Silver oxide. Copper nn'n T9 the bei@ ingredients., are added nlitalil.l?.`

lining agents such as antiniony oxide, arsenio'ox idejor sodium chloride in an amount ranging apthat the vsilica contentv should be .provided in amounts ranging between 60% and 72% since when using .more than 72% the batch -will tend to be viscous while :less than 60% will result in .a glass which is not chemically durable.

The boric oxide is used to help in the melting by making the glass a little more vvliu-icl and tending to improve the chemical durability. However, it should be held to not .more than 7% since larger amounts produce undesirable characteristics in the glass.

Soda and p otash are included lin amounts ranging approximately from 14% to l20%, part as carbonate and part as nitrate, 'although only. nitrate lmay be used, if desired. However, such an alkali should be controlled in accordance with be held to approximately between 9 and 11%'.

However, the lead oxide may. :be substituted in Whole for the lime if desiredalso, fliine can he used without any lead oxide. 'Combining lime? with lead oxide improves the .chemical sta-bility of the glass.

-Io'vveven since the-lead oxide content is responsible for producing the desired X-ray absorption, it is necessary to include it inr amounts of at least `3% 'PbO whereuponthe resultant glass! will absorb substantially all the Xfrays generated ny 40,000 volt electrons. This is sufficient ab.-Y sorption for present purposes since. it is believed that the present invention .will generally -be lused- Witl'i lowei'electron voltages such as the .conven-v tional television -lgincscope tubes 'which' generate a maximum of approximately 36,000 voli'l eleo-- trons.

The ymanga; lese oxide and vanadium o xidetof. 'g'etner .control .the visible color and the amounts used must be carcfullycontrolled. The vanad-iurn oxide should be provided in an amount in accords.. ance with the ultrafviolet .absorption desired.

'Then the manganese oxide is added toget a sub.

.siantially neutral colored glass. 'Since it lias-.been' found that manganese oxide, when used in subs.'

stantially large quantities, will produce aA glass.' h'aving'a transmission curve exhibitinga marked.:v dip in the center or yell-owfgreen region, itis preferable 'to provide the manganese: oxide greatcrquantities than the vanadium oir-ide. to get the desired balance and produce'a glass having# the desired transmission curve, The manganese. oxide may be varied .approgtiniately f rornijtc-l-Z; times the amount of vanadium oxide depending range 'of fromopljrpxllla'tely 0.05% M -0.15%.

It has been ifo-.1nd, however, that a more level curve can be made to exist in the visible specs.. trum by including controlled amounts of copper @.Xse Lathe belimlxls? White will producen,

' The cur-ve 'transmission of the glass of batch A. 'curve i0 of batch B, curve H o f batch C,curv'e l2 of .batch D,

and .curve [3 of batch E.

rBatches F through M were melted at sorre- 5 sponding time-temperature cycles.

indicated by rmrneral'.i6V yin Fig. 6 indicates the resultant visible transmission curve of the glass Large of batch F, numeral 210i batch G, num eral 28 of It has/also been found that as a. separa-te lter l5 supported by a suitable y of a television kinescope tube is lowered by the' utlra-violet radiation from the kines'cope, par.- tieularly tn'e ult-raviolet radiation below 365 mu.

tube of a glass having ultraeviolet absorption characteristics in accordance with the present teachings, this eliminates the harrn'iy upon the observer'seyes and 'results in mainer at substantially the normal level whereby the observer is able to watch the kinesoope with greater ease, less eye strain, and less fatigue. The absorbing glass may. of cou-rese, be eficient-` other-wise superimposed upon the face' po 3 of the tube. as shown in Rig. 3, or mayv be .made

Vautag cri them readily.

.g. 5. The cures renders glass having less l.transmissie'n `in the red region of the spectrum.

additions to the batch I nixture of silver oxide red'uces the ultrawlolet. transmission Withfsubstantially littleif any eect upon the visible spectrum,

Copper oxidel can be inclu-:led in amounts ranging approximately from 0.045% to 0.4% and silver oxide approximately from 0.1% to 0.2%.

Glasses produced from the above. formulae have outstanding characteristics with respect to' all the practical criteria, the chemical durability Therefore, by making the face portion 3 of the is good, and the glasses have a desirable co-eiliy cient of expansion which In produei g glasses from the above batch mixes. the melting can be done in a relatively 60 'tenance of the threshold of vision 'of the observshort time at temperatures ranging between 2600 to 27755` F. or the temperatures can be dropped by 50 to 100" F and the batches mel-ted for a length of t 1e ranging from 6 to 21/2 hours. It will be noted, however, that the batches 'are 65 ly used as a laminate i4 .cemented as at t5 or melted under oxidizing conditions in airy conven-A tional induction electric, gas-red, or other type of furnace. The resultant glasses produced by melting the fusible with other stable glasses.

batclxesA through D at temperatures held be- 70 tween 2635-2700 F. for 11/4 hours, and batch E at temperatures held between 26500-2700.o F. for 1 hour 35 min., will possess visual transmission characteristics as illustrated i 'indicated by numeral 9 illustrates the 'visual 75 'ressent screen of the tute. The color can be .amaca wavelength. Such. a coating A8, due to the ex- Atre'rnely small size of its particles and irregularitiescompared to the relatively large size of the particieso! the fluorescent screen 1, will cause the screen l to be spaced from and consequently out of optical vContact with Athe glass 3. Thus, the reii'cctions 22 will be still further prevented `from illuminating the .screen 1.

The outer coating 5 also aids in providing com- Afortable viewing of the cathode ray -tube by an .observer by reducing reflection into'an observers eyes of light rays emanating from sources outside the tube.

Since light rays 24, Which'einanate from the .particle i3 and strike the surfaces of the glass at angles greater than the critical angle will continue on through the glass Without emerging, it is further desirable to provide mea-ns on .the outer periphery of the face portion 3 for rendering the rays 24 ineffective. Such means `may be in the form of a highly polished' sur :face for permitting' the rays 24 to escape suo stantially without di'usion, or in the forni of a coating 25 of light nbsorptive material for ab- Such an absorptive coatsorbing the rays 2d. ing may be` a suspension of graphite particles in ,oil or cement such as Canada balsa-m, or black lacquer, which coating should possess an index characteristics, color, and ultra-violet and X-ray absorption characteristics oy combining controlled amounts of the oxides of Hmanganese, vanadium, silver and copper together with lead oxide in a base crown glass batch and melting under oxidizing condi-tions lWithin the temperature-time lcycles given herein. The resulta-nt glass will, when used in connection with cathode ray tubes, increase contrast discrimination of the image produced on the screenof the tuba absorb ultra-violet radiation and thereby'vmaintain the threshold of vision of an observer at substantially normal level, labsorb hdmi-ful X- rays, and transmit substantially true colorl and when used with reflection reduction' sur-lace coatings, with or without peripheral lig'ht eliminating meanspwillgreatly reduce halation and thus further 'increase contrast discrimination in the image.

From the foregoingit will he seen that we ,ia-ve produced means and methods of a simple and eicient nature for producing all of the objects and advantages of the present invention.

It will be understood. however, that many changes may be made the compositions, structures and methods shown and described-Without departing from the spirit of the invention as expressed in the accompanying claims, and it is. therefore, not desired that the compositions,

structures and methods deseribed'be limitive of" the invention since the preferred only have -been given 'oy trayl of illustration, A

We claim:

controlledmultra .iipletaby ticsani f'a controlled color, s hang-made f1-o1xa-lihfjilxreon sentiallvj, oi ,soda-potash-lime-silice. ig incorporated therein from about .05 ci' vanadium oxide andforn about 0.4 to

' 1.4% of manganese oxide, the 'sodal and potash content together comprising from about 1i to 1o Y 20%. the irme content not eircceemg, 11% and .the silica content embodying .at Estli.

said glass, forja thickness of approximately 6v millimeters, permitting transmission of les: than 13% of ultra-violet at 360 minimis-ronnie; the spectrum and visual transmission` of am.' mately from 45%' to '70% at 400,

56%to 76% at 500 mu, 50% to 71% at mu, .and 6 7 'It 1:08092, at '700 mu, with an averagewhte or' approximately -from 60% to 75%. l 2. A glass having vcontrolled .ultra-viola ahsorption characteristics and of a controlled-color, said glass being made from a batch ing essentially of 'approximately Lrorrrltlji'r @72% of silica, approximately from 0% to 7% of fmlc oxide, approximately from 14% to 20% Gf alkali oxides, approximately from 9% to 11% of amixture of lime and lead oxide, approximately from 0.5% to 0.2%.-of fining agents, and the oxidase! manganese and vanadium in amounts controlled in accordance with the transmission color characteristics desired of the resultant glass, with the manganese oxide being in amounts ranging .approximately from 4 vto 12 times the amenait of vanadium oxide and the vanadium oxide comprising from about .05 to .15%, said glass leaving trans-mission characteristics for a thickness of approximately 6 millimeters which permits transmission of less than 13% of ultra-violet at 36D' X-ray absorption characteristics 'and of e. mn-

trolled color, said glass being made from a hatch mix consisting essentially ofl approximately rpm 60% to 72% of silica, approximately from @3% eo 7% of boric oxide, approximately from ls to 420% ofalkali oxides, at least 3% of lead from approximately 0% to 8% of lime, approd- Inately from 0.5% to 0.2% of ning agentsY and manganese oxide and vanadium oxide in amountsY controlled in accordance with the transmision and color characteristics vdesired of the glass, with the manganese oxide being in amormts ranging approximately from 4 to 12 the amount of vanadium oxide, and the vanadium oxideooinprising from about .05 top :saidl glass lia-ving transmission characteristics-fur mix consisting essentially oi' approximately 60% to 72% of silica, approximately from et@ to 7% of boric oxide, approximately from 14% to 20% oi' alkali oxidesat least 3% of lead ori-Freie,

approximately 0.5% to 0.2% of inning am. Y from about 0.4 to 1.4% rif-manganese oxide and' from about 0 .05 to 0pl5% 'of vanadiummirsaid glass having transrniss-ionl characteristics for a thickness of approximately .fi millimeters which ,Y

permits substantially no transmission or' maltravioiet below approximately 34e' in the spectrum andk visual transmissionof mately from 45% to '10% at 400 millimiezons. 56% to 76% at 500 mu, 50% to 70% at '600 .11111. and`56% to 80% at 700 mu, with arravcr'age white oi' approximately from 60% to 70%, and 'substantially no transmission of -rays generated by 40,000 volt electrons. I

5. A glass having controlled ultraaviolet and sii-ting essentiallyoisilica, so,da, potash, and lime in proportions sulcie'nt to form a glass and wherein thefzlica. comprises at least 60% *of the n -whole from .U5 to .15% of vanadium 'oxide and as large as the' .amount .of-vanadium oxide, and" melt-ing said batch onder oxidizing conditions, they Xvray absorption characteristics and of a controlled color,- said glass being .made from a batch mix having incorporated therein approximately 67.8% of silica (S102) approximately 4% of boric oxide (B201), approximately 16%' o! sodium ar'id potassium oizides (Naro and KzO) approximately 1% of lime (Caill), approximately 3% of lead` oxide (PbO), approximately 0.6% of manganese oxide (MnOz), approximately 0.15%'01 vanadium oxide (V295), and approximately 1.5% of anti` mony oxide-($13203). said glass being otsoch character that at a thickness of approximately 6.37 millimeters it transmits in the visible spectrirm approximately 54% at ti0() millimicrons, "71 at 500 millimicro'ns, 67% at SGO-millimicrons, and '79% at '700 millimicrons, land transmits in the `ultra-violet regionofthe spectrum approxi.-

mat-ely 36.5% at 390 millihicrorls, 21.0% at 38.0

millizniorons, 7.2% at 370 millimicron's, 1.0% at '360 millrnicrons and substantially nothing beow 350' millim-icrons.

6. A glass having controlled ultra-violet and X-'ray absorption characteristics and of a controlled color, said glass being made from a batch mix having incorporated therein approximately 67.8% of silica (S102), approximately 4% of boric oxide (B203), approximately 16%.01' sodium and potassium oxides (NazO and KzO) approximately 7% of lime CaO)', approximately 3%' of lead oxide (PbO), approximately 1.0% of manganese oxide (M1102), approximately 0.15% of vanadium trollcd color, said-glass being made from a batch mi?. having incorporated. therein approximately 67.8% of silica (SiOg) approximately 4% of boric oxide (B201), approximately 16% of sodiumV and potassium oxides (Na-O and B220) ,approximately of lime (CaOl, approximately 3% of lead voxide (PBO), approximately 0.6% of manganese oxide (M1103), approximately 0.1% of vanadium oxide (V205), and approximately 1.5% of anti- `mcny oxide (813203), said glass being of suf-h chai"- aeter that at a thickness of approximately 6.33 millimeters it transmits in the visible spectrum approximately 62% at 400 millin'1icrons,`72% at soo minimis/rens, 68% at coo mmimicrons, and

19% at 700 millirnicrons, and transmits in the 1Iltrawiolet region of the spectrum approximately 45.6% at 390 millimicrons. 32.1% at 380 millimierons, 17.1% atr 370. millimicrons, 5.0% at 360' millimicrons 0.7% at 350 millimicrons. and substantially nothing below 34.0 millimicrons.

8. The method of makingan ultra-'violet absorting glass having a greenishblue color comprisng the steps. of adding to a glass hatch cor.-

istics as given for a thiol-mess of approximately an amc-unt of manganese oxide from 4 to 12 times proportions of the vanadium oxide and manga.-

nese oxide being such as to produce in the resultarxt glass the desirable color and transmission characteristics similar to the follow/fing transmission vcharacteristics as given for a thickness of approximately six millimeters and'which permits less than V13% ultra-violet transmission at 360 millim'icrons in the spectrum, and visual transmission of approximately from 45% to '70% at 400v millimicrons,56% 75% at 500 milliznicron's, 50% to 70% at 600 millimicrons, and 66% to 80% at 700 mill-imicrons, and an average white o! approximately 'from 50% to 70%.

9. The method of makingan ultra-#violet and X--ray absorbing glass having a greenish-'clue .colorfcomprising the steps of adding to a gia-SS batch consisting essentially of silica, soda, potash and lime in proportions sufficient to form `a glass and wherein thesilica comprises at least 60% of the whole at least 3% of lead oxide, and from .05 to .15% vanadium oxide' and an amount ofC manganese oxide from 4 to 12 times as large as the amount of vanadium oxide, and melting said batch under oxidizing coriditicn for a controlled time-temperature cycle so vas to produce in the resultant glass the desired color and X-ray absorption and transmission characteristics similar to the following transmission cliaracte-ifstics as given for a thickness of approximately six milk-- meters and which permits substantially notransmission ofrultra-violetbelow approximately 340 millirnicrons in 'the spectrum, and Visual trans mission of' approximately from 45%to 65% at' L100millhnicrons 62% to 72% at 500 millimiorons, 59% to 69% at 600 milliniicrons, and 75% to 80% at' 700 mil-li-microns, andan average white of approximately from' 60% to 70%.

10. The method of making an nltra-y'iolet ab? Sorbing glass having a greenish-blue color comprising the steps of adding to a glass batch con,- sisting essentially of silica, soda, potash, boric oxide, and lime in-proportions suiiicient to form a glass and wherein the silica. .comprises at least- 60% of the whole from -05 to .15% of vanadium oxide and en amount of ma-nagr-mse oxide from 4 to l2 times as large as the amount of yanadiomoxide, and -meltingjsa'id batch under oxidizing conditions at aeontrolled temperature Within the range of from 2000u F. to 2775 F. and the proportions of vanadium oxide and manganese oxide being such as to prod-'nce in' the resultant glass' the desired color and transmission characteristics similar to the follow-ing transmission charactersin; millimeters and which permits less than 13% "ultraviolet transmission at SGO-millimlcrons in the spectrurmarid visual transmission of approximately from 45% .to 70% at 400- mi-ilix'nicrons. 52% tosz, at 5.00 minimis-roos, 50% 'to 70% v'at' microns, and ai; average White of approximately 1.7011126070 to 70%.

11. The method of making an ultraavi'olet and.

X-ray absorbing glass having .ay slight greenisl1- lue color comprising thesteps 1f-forming a batch cor'itainirvT approximately 67.8% of silica (SiOa),

approximately 4.0% oi boric oxide (13201). approxmately 16.0% ofsodium and potassium (Mnpg), approximately 0.15% of vanadium oxide (V2Os),` and approximately 1.5% of antimony oxide (SbzOg) melting said batch'under oxidizing conditions in a fummo -for `pproximately 1l/4 hours at a- `temperature maintained between 26.95 F; and 2700 F. so as to produce in the resultantglass at a thickness of approximately six millimeters kthe desired X-iay absorption and color transmission in the visible spectrum of ap- 70o mu m the introspection with an average l f proximately'54% at 400 milliirxicrons, 71 %=at 500 A millimicrons, 67% at 600 milliin'icrons, and 79% at 700 millimicrons, vand transmission in the ultra-violet region of the spectrum of lapproximately 36.5% at 390 millimlcrons, 21.0% at 380 millimicrons, 7.2% at 370 millimicrons, 1.0% at 360 milliinicrons, and substantially no transmission below 350 millimicrons.

12. T he method of making an ,ultra-violet and X-ray absorbing glass having a Slight greenish- 'blue color comprising the steps of forni-ing a batch containing approximately 67.8% of silica (SiOz).

approximately 4,0% of 'boric oxide (B203), apfproxirnately 16.0% of sodium and potassium oxides (NaaO and KzO) approximately 7.0% of'lime (CaO), approximately 3.0% of lead oxide (P110),

Iapproxirn'atoly 1.0% of manganese oxide (MnO) approximately 0.15% of vanadium oxide (V205), and approximately 1.5% of antiinony oxide (SbzOa) melting said batch under oxidizing com ditions in a furnace for approximately 1% hours at a temperature maintained between 2635 F. and 2700" F. so as to produce in the resulta-nt glass at e. thickness of approximately six\milli meters the desired color transmission inlthe `vi si'- ble spectrum of approximately 50% at e0-0 millimicrons, 63% at 500 millimiorons, 59% at 600 milliniicrons, and 75% at'lOO millimirons, and transmission in the ultra-violet region of `the spectrum of approximately 32.8% at 390 milli-A microns, 18.6% at 3180 muli-microns'. 6.6% at '370 'mll-microns, 0.7% at 360 mill-imicrons, and sub stantially no transmission below 350 millimicrons.

13. Anultrawd'olet absorbing glass suitable for use as a cathode ra'y tube face, Iilter and the like comprising the fused product of a I nixture of silioapsoda, potash and lime insufficient proportions to form a substantially.homogeneous glass, said mixture further containing vanadium and 'manganese oxides wherein the' proportion of VVmanganese oxide is from four to twelve times the proportion of vanadium oxide, and the vanadium oxide embodies from about .05'to .15 per cent of the whole mixture.' said -glass for a thick-ness of approximately 6 millimeters permitting substantially no transmission of ultra-violet below 340l mu and about 45 to 65% at 400 mu, 62 to 72% at 500 mu, '9-to 69% at 600 mu, and 75 to 80% at WhiteA of' aiprmzirnately from 60 `to '10%.

14. An ultra-violet absorbing 4glass suitable for use as a cathode raytube' `face, filter andthe like comprising' the fused product of a mixture of silica, soda. potash'and 'lime in s'ufci'ent proportions to form a substantially homogeneous'gl'ass .and further containing from about 0.4% to 1.4% 'of manganese oxide and frornabout 0.05 to 0.15% of vanadium oxide, said glass for a thickness of approximately 6 millimeters permitting substan tially no transmission of uitraevi-olet below 340 mu and about 45 to 65% at 400 mu, v(i2-to 72% at 500 mu, 59 to '69% at 600 mu, and'75 to 80% at 700 mu in the visible spectrum, `with an average white ofapproximawly from 60 to 70%.

the.iixsaeomonsinajneiusedlnmfun@ o lead'oxide is atleast 3% of the wholemixture, ythe asv proportion of manganese-oxide is from four to twelve times the proportion of yanadium'oxide, and the vanadium oxide embodies from. about .G5 to .15 per cent .ofthe whole mixture, said glass for a thickness of approximately 6 millimeters permitting substantially no transmission of ultra..-4

violet below 340 Inu'and about 45 to 65% `at; 400- 'mu, 62 to 72% at 500 mu, 5,9 to 69% at 600 mu, and 75 to 80% at 700 mu. in the visible spectrum, with an average White of approximately from '60 to .16. An ultra-violet,l Xeray absorbing glass suit.- alile for use as a cathode -ray tube face, lter and the like comprising the fused product of a mix` ture of silica, soda, putas-h, boricl'oxide and lime in suiilcient proportions tufo-rm a substantially homogeneous glass, said mixture further containing from about .05fto' .15% of .vanadium oxide.r

from about 0.4 to 1.4% of"maiganese oxide', and-- Referee-ees one@ in eine sie of uns patent -.`UNITED'-'.STMS.PATENTS Number Name 'Data y 1,292,148 Taylor..so ..-....dan. 21, 1919 v0,219,122 Weidert t- Get.`22, V1940 

1. A GLASS HAVING CONTROLLED ULTRA-VIOLET ABSORPTION CHARACTERISTICS AND OF A CONTROLLED COLOR, SAID GLASS BEING MADE FROM A BATCH MIX CONSISTING ESSENTIALLY OF A SODA-POTACH-LIME-SILICA BASE HAVING INCORPORATED THEREIN FROM ABOUT .05 TO .15% OF VANADIUM OXIDE AND FROM ABOUT 0.4 TO 1.4% OF MANGANESE OXIDE, THE SODA ND POTASH CONTENT TOGETHER COMPRISING FROM ABOUT 14 TO 20%, THE LIME CONTENT NOT EXCEEDING ABOUT 11% AND THE SILICA CONTENT EMBODYING AT LEAST 60%, SAID GLASS, FOR A THICKNESS OF APPROXIMATELY 6 MILLIMETRES, PERMITTING TRANSMISSION OF LESS THAN 13% OF ULTRA-VIOLET AT 360 MILLIMICRONS IN THE SPECTRUM AND VISUAL TRANSMISSION OF APPROXIMATELY FROM 45% TO 70% AT 400 MILLIMICRONS, 56% TO 76% AT 500 MU, 50% TO 71% AT 600 MU, AND 67% TO 80% AT 700MU, WITH AN AVERAGE WHITE OF APPROXIMATELY FROM 60% TO 75%. 